There is a wide variety of peptides in blood. These peptides include peptides that clearly indicate the differences in concentrations in blood between healthy and pathologic conditions. Such characteristic peptides are regarded to be biomarkers for diseases and to be beneficial to clinical test. For example, it is known that the concentration of ghrelin in plasma decreases in the case of patients suffering a gastric cancer associated with serious heart failure and severe cachexia. Because the concentration of brain natriuretic peptide (BNP) in blood is important as a clinical index of heart failure, the peptide is used as a test marker.
There is a large amount of protein referred to as “albumin” in blood. The albumin protein is known to inhibit detection of peptides in blood. In order to remove albumin from a sample of blood, a method has been conventionally performed in which the blood sample is loaded onto a column adsorbing specifically albumin. Albumin in the blood sample remains on the column, while a liberating form of peptides pass and were collected. (Lowenthal M S et al. “Anaysis of albumin-associated peptides and proteins from ovarian cancer patients”, Clin. Chem., vol. 51, 1933-1945 (2005)) and Zhou M et al., “An investigation into the human serum interactome”, Electrophoresis., vol. 25, 1289-1298 (2004)).
However, the recent reports have showed that most peptides in blood bind to albumin to form a complex. For example, Lowenthal et al. have disclosed 98% of peptides in serum are bound to and complexed with albumin. That is, by the column method of adsorbing albumin and passing peptides, peptides undesirably remain with the column-adsorbed albumin and were abolished upon disposal of the column. Consequently, only a very small amount of the peptides are obtained.